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The number of knee replacement surgeries has risen steeply in recent years.

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Getting a new knee because the original has worn out and may have become painfully arthritic is an increasingly common surgery in America. First-time knee replacement surgeries rose 63 percent between 1997 and 2004, according to a 2008 paper in Arthritis Care and Research. If that clip keeps up, some 1.4 million surgeries will be performed in 2015, researchers estimated.

Findings coming out of the 2010 American Academy of Orthopaedic Surgeons annual meeting in March are shedding helpful light on knee replacements, including how active one can be with an artificial knee, how young or how old one should be to undergo the surgery, and the varying benefits to be gained.

When patients and their doctors decide whether to proceed with a knee replacement, “it’s always a quality-of-life issue,” says Mark Figgie, chief of the Surgical Arthritis Service at Hospital for Special Surgery in New York. Osteoarthritis, which causes the cartilage in joints to wear away, is the usual culprit when knee pain has made walking painful.

Even sleeping can be very uncomfortable — sometimes unbearably so — because a day’s worth of moving around has inflamed the joint. And being unable to enjoy the activities you once did — say, golf or cycling—also can be a quality-of-life factor, says Figgie.

Whereas patients used to have knee replacements in their late 60s or later, Figgie says he now is more often doing the procedure in folks in their 50s — and not infrequently, in patients in their 40s. The standard practice used to be that younger patients with knee trouble would be prescribed painkillers or be given steroid injections for several years to tide them over until surgery.

Indeed, information presented at the March AAOS meeting found that the average age for Americans getting total knee replacements has declined. Comparing the years from 1990 to 1994 with 2002 to 2006, researchers learned that the average age has gone from 70 years to 68.

Experts see several forces at work.

“Now we’ve got better materials and we’re more comfortable doing [total knee replacements] in patients younger than 60 or 65,” says Michele D’Apuzzo, an orthopedic surgery resident at the Mayo Clinic, Rochester, Minn., and lead author of the study. In particular, the plastics being used to replace the worn-out cartilage and serve as a cushion in the knee joint have improved.

Still, the younger patients having these surgeries will likely live another 20 to 30 years, and the data on how well these improved components will last, says D’Apuzzo, is “not there yet.”

In addition, folks’ expectations of continued mobility and activity have risen. Not only do aging baby boomers expect to keep doing the things that can really put pressure and strain on joints — like tennis, skiing, or jogging — but younger people, too, don’t want to wait for a knee fix if the technology is available now.

Research presented at this year’s AAOS meeting suggests, however, that even the activities that the Knee Society, a professional group for knee specialists, currently considers a no-no after knee replacement may be OK for some individuals. These activities are ones that pound on the knee and put the most pressure on the joint, like singles tennis, running, soccer and football.

“We were expecting patients who practiced impact sports would have terrible complications,” says Sebastien Parratte, an orthopedic surgeon who was doing a one-year fellowship at the Mayo Clinic when the study was performed. That was not the case, however. The results showed no significant differences after 7.5 years of follow-up in patients who stayed away from the not-recommended sports and patients who played them anyway.

The surprising results, Parratte says, were “reassuring, but it’s only one study. It’s not enough to change the recommendation.” He notes, however, that while the Knee Society says patients can do less punishing activities, like cycling or swimming, they are not officially recommended. Perhaps it’s time to consider recommending such gentler activities, says Parratte, since the subjects in the trial that fared best after knee replacement were the least likely to be obese or have diabetes or cardiovascular disease.

“They are healthier and practice sports,” he says. Indeed, Figgie notes that generally speaking, people who do best after a knee replacement are in good shape: “It’s better to be fit than fat,” he says.

And older knee replacement patients may reap the added benefit of improved balance, according to other research coming out of this year’s AAOS meeting. Israeli researchers found that subjects with an average age of 73 who had total knee replacements enjoyed significant improvements in balance, pain and self-reported quality of life. With age and “as patients get more and more deformity [in the knee joint], it gets more painful,” says Figgie, who was not involved in the research.

As the study showed, he adds, post-knee replacement patients again have a straight knee and the ability to fully flex the joint. This provides them more stability, which improves their walking and, of particular importance to older folks, reduces the risk of falling. As long as patients are healthy enough to withstand the effects of surgery, he says, they are not too old to have a shot at improved quality of life with a knee replacement.

Protect your knees

Nearly half the adults in America will develop osteoarthritis by the time they’re 85, and obesity will be the main culprit. You can protect your knees by staying active and strengthening the right muscles (at any age) but take these precautions:

• Lighten the load. Simply walking around puts pressure equal to three to five times your body weight onto your knees, and toting around extra pounds makes the burden even worse. Excess weight on your knee joints may accelerate osteoarthritis, the degeneration of the joint, so shed some of those pounds.

• Work all the muscles. Strengthening and flexibility exercises can build up muscles to stabilize the knees—but don’t bulk up one group and forget about the others. Most important are the quads (front of thigh), hamstrings (tendons and muscles behind the knee and thigh), and hip abductors and adductors (outer and inner thighs, respectively).

• Put it on ice. Don’t ignore a tweak or strain, no matter what sport you take up. This is a simple way to reduce inflammation and pain.

• Avoid rotation of hips and knees. Jumping and slowing down from a run — common to many exercises— seem to be particularly bad for the knee’s ACL (anterior cruciate ligament). Land with joints aligned so that hips are over knees, knees are over ankles, and ankles are over toes — but with some give to the joints.

• Beware the up and down. Running in hilly terrain can put particular strain on tendons in the knees. Pay attention to clues that your knees need a rest—or at least a flatter course.

• Cross-train. Overuse is the slow and steady way to knee injuries. Opt for a range of varied activities by alternating workouts.